The last time someone tried to build a hockey arena in Allentown, the risk of sinkholes and cost of dealing with them helped sink the project.

That was a decade ago.

Work crews have begun drilling test holes deep beneath the surface at the site of the new proposed arena, just two blocks away at Seventh and Hamilton streets.

What they find beneath will have a significant effect on construction.

The more precautions the city has to take to deal with the type of cavity-pocked, sinkhole-prone limestone rock formations that are common to Allentown and the Lehigh Valley, the higher the construction costs, experts say. Not that there's an alternative.

"You've got to work with the geology you've got," said Jonathan Nyquist, chairman of Temple University's department of earth and environmental sciences.

The last major structure built on the property — the seven-story Corporate Plaza office building — fell victim to a pair of sinkholes in 1994 and was demolished.

City spokesman Mike Moore acknowledged Monday the city is performing standard pre-construction soil testing, but would not comment on a timeline or when demolition is expected to begin.

The city has acquired more than two-thirds of the 34 properties it needs to build the 8,500-seat arena for the minor league Phantoms, the Philadelphia Flyers' top affiliate. The estimated cost of construction is more than $100 million.

The Lehigh Valley is laced with sinkholes. Just ask the PennDOT engineers who battled them as the agency built the Route 33 extension. One needs to go no farther than downtown Bethlehem to see how unexpected underground conditions can complicate a construction project.

When work began on developer Lou Pektor's $14.5 million One East Broad office building in 2005, unforeseen voids or gaps in the rock beneath the ground quickly sucked up Pektor's entire $230,000 contingency fund. He was forced to fill them with concrete to stabilize his foundation.

But stadium supporters can take heart. The Ninth and Hamilton streets location selected in 2000 to be home to the Lehigh Valley Extreme hockey team and then abandoned? It's now home to the PPL Plaza office building. The company hired a soil stabilization firm that drilled 357 holes at the site and pumped in concrete "grout" to fill the limestone cavities beneath..

That's one of a variety of steps that can be taken to create a stable foundation, said Chris Taylor, senior geologist at Hanover Engineering Associates, but first you have to know what kind of earth and rock you are building it on. Voids in porous limestone can weaken the bedrock.

"The important thing to realize is that man-made improvements have a certain weight to them," Taylor said. "That weight has to be supported by the natural material underneath: the soil and the bedrock."

Large buildings can be built with pilings that anchor the structure deep in solid bedrock to provide additional support, Taylor said.

But even deep pilings can be vulnerable to large sinkholes, said Sibel Pamukcu, interim chairwoman of the department of civil and environmental engineering at Lehigh University. Grouting cavities can be effective, but the costs can escalate because it's difficult to know in advance how much concrete will be needed.

Taylor agreed: "Every yard of concrete, every bucket of soil that has to come out costs money."

Precautions can be taken to lessen future risk, but there's almost no way to completely prevent future sinkholes, Taylor and Pamukcu said.

At the arena site Monday, workers with Cinnaminson, N.J.-based FM&W Drilling bored down 10 to 50 feet, taking soil and rock samples.

Another contractor used a backhoe to dig beneath the worn macadam surface to examine the contents of the fill that was used to level the site. The front-end loader's steel scoop unearthed piles of earth, rock, concrete, brick and twisted metal re-bar used in reinforced concrete.

If the fill is full of debris, the builder will probably need to replace it with a more homogenous soil that won't shift and will compact in predictable ways when the arena is built, Taylor said.

There is some debate in the engineering community over whether it makes sense to build on limestone formations at all, Taylor said, because sinkhole formation is unpredictable. But because most of the Lehigh Valley sits on top of the porous rock, there's really no way around it.

"If you want to be down in the Valley, you are going to be on limestone," Taylor said.